# Technical Glossary

Phoenix Legacy defines many concepts in the core contract and SDK. They are defined in the code and here below.

### Exchange Language

* **base**: Token traded in a market (e.g. SOL in the SOL/USDC pair).
* **quote**: Token used to price the base token (e.g. USDC in the SOL/USDC pair).
* **bid**: A limit order on the order book to buy base token with quote token. The amount of each respective token is specified by the price and size parameters of the order.
* **ask**: A limit order on the order book to sell base token for quote token. The amount of each respective token is specified by the price and size parameters of the order.
* **swap**: An order that crosses the book, usually immedate-or-cancel (IOC).
* **seat**: To place limit orders on Phoenix Legacy, a user must have a seat in the relevant market. Seats are assigned by the market authority.
* **lot**: The discrete token unit that the market uses.
* **tick**: The discrete price increment that the market uses.
* **market sequence number**: The sequence number of a market. Each instruction interacting with the market increments the sequence number.
* **order sequence number**: The sequence number of an order. Each limit order placed on the book gets a monotonically increasing order sequence number. The side of the order (bid or ask) is encoded in the order sequence number's first bit: 1 for bid, 0 for ask. See `Side::from_order_sequence_number`.

### **Units**

* **decimal**: For a token, the decimal precision in which quantities are measured. For example, USDC has 6 decimals so the smallest positive quantity of USDC is 0.000001 USDC.
* **atom**: A token quantity measured in terms of its highest precision. For example, 0.01 USDC is equal to 10000 USDC atoms.
* **unit**: A token quantity measured in its usual representation. For example, 100 USDC is equal to 100 USDC units.
* **lot size**: The number of atoms in a lot. For example, if a lot is 0.01 USDC, then lot size is equal to 10000.
* **tick size**: The price increment that the market uses, in quote lots per base unit. For example, for a SOL/USDC pair, if a lot is 0.001 USDC (lot size is 1000) and the price increment is 0.01 USDC per SOL, the tick size is 10 (quote lots per base unit). A price of 15 USDC per SOL is 1500 ticks.

The Phoenix Legacy units system is fairly complex. For a more detailed explanation, check out the deep dive into the unit system on the [next page](https://ellipsis-labs.gitbook.io/phoenix-dex/tRIkEFlLUzWK9uKO3W2V/getting-started/technical-overview/units).